Eye of Sauron

"But suddenly the Mirror went altogether dark, as dark as if a hole had been opened in the world of sight. . . In the abyss there appeared a single Eye that slowly grew, until it filled nearly all the Mirror. So terrible was it that Frodo stood rooted, unable to cry out or to withdraw his gaze. The Eye was rimmed with fire, but was itself glazed, yellow as a cat's, watchful and intent, and the black slit of its pupil opened on a pit, a window into nothing."

The Eye of Sauron, called by many various names, was the symbol of Sauron the Dark Lord. This symbol was adopted to show his unceasing vigilance and piercing perception, and was displayed on the weaponry of his servants, or at least the orcs.

In Peter Jackson's film adaptation, the Eye featured as a physical, living object between the pinnacles of Barad-dûr. This is usually interpreted as Sauron's actual physical form during the War of the Ring, however, in the extended edition of Return of the King, there is a scene where the camera zooms into the pupil of the Eye, briefly revealing Sauron in his humanoid, armored form. The Eye also literally searched the lands with a red beam of light like a spotlight, and in the film the light could be seen constantly sweeping the lands. It was quenched when the One Ring was cast into Orodruin.